I'm having the same problem with the bump steer thread but I believed my error message when it told me that it was my cache administrator
Anyway, I was never really sure how to measure rear castor. Is it along the plane of the reverse-a-arm or do you use an imaginary line drawn vertically along the centre of the upright or what? And is it relative to the ground or the chassis? There are many other options too and anyway, however you measure it, it changes dynamically doesn't it? Someone once even insisted that the term “rear castor” is meaningless because without steer there is no castor (but I think we all know what we mean).